The present invention relates to a method for using data obtained by transmission of penetrating radiation through an object to enhance an image of penetrating radiation that has been scattered by the object.
X-ray radiography applied to the non-invasive inspection of the contents of containers is a mature technology carried out by a wide variety of methods. An early purpose of x-ray radiography was to produce a high resolution projected image for visual inspection of the contents. In the past decade, as the purposes of the inspection have become targeted to finding specific contraband such as drugs and explosives, methods of dual energy and backscatter radiography have been developed to measure the atomic number of the objects, as have tomographic techniques to measure the density of the objects and coherent scattering methods to measure the crystalline properties of objects.
As used in this description and in any appended claims, the term xe2x80x9cimagexe2x80x9d refers to any multidimensional representation, whether in tangible or otherwise perceptible form or in a computer memory or a storage medium, whereby a value of some characteristic is associated with each of a plurality of locations corresponding to dimensional coordinates of an object in physical space, though not necessarily mapped one-to-one thereonto. The term xe2x80x9cimagexe2x80x9d includes an ordered representation of detector signals corresponding to spatial positions. For example, the image may be an array of values within an electronic memory or holographic medium, or, alternatively, a visual image may be formed on a display device such as a video screen or printer. Thus, for example, the graphic display of the spatial distribution of some feature, such as atomic number, in one or more colors constitutes an image. Similarly, xe2x80x9cimagingxe2x80x9d refers to the rendering of a stated physical characteristic in terms of one or more images.
Backscatter imaging, in which x-rays scattered by a material in a direction directed generally backward toward the source of radiation are employed, offers several unique inspection capabilities and operational features known in the art. For example, because the scatter signal falls off quite rapidly with increasing depth into the object, backscatter images effectively represent a xe2x80x9cslicexe2x80x9d of the object characteristic of the side nearest to the x-ray source; this image is frequently useful even when a transmission image representing the same scanned area is hopelessly confused by image clutter.
The underlying physical phenomenon that leads to scattered radiation is the Compton effect. Low atomic number (low Z) materials, which encompass organic materials, interact with x-rays principally by Compton scattering. Narcotic drugs, being among the densest of organic materials, tend to produce the brightest signatures in a backscatter image, as do organic explosives, making backscatter imaging a useful imaging modality for bomb or drug detection.
Backscatter imaging relies upon the direct detection of photons which have been Compton scattered. In the prior art, an image is created that is separate and independent of any transmission image that may be produced at the same time.
In accordance with one aspect of the invention, in a preferred embodiment, there is provided a method for displaying an image of contents of a container. The method has a step of illuminating the container with at least one beam of penetrating radiation and a step of detecting radiation scattered from the beam of penetrating radiation by the contents of the container for forming a scatter image of the contents of the container. The scatter image is characterized by a plurality of scatter pixels. The method also has a step of detecting radiation from the beam of penetrating radiation transmitted through the contents of the container for forming a transmission image of the contents of the container, the transmission image having a transmission pixel corresponding to each scatter pixel of the scatter image. The method then has a step of forming a combination image having pixels in such a manner that a value is associated with each pixel. The value that is associated with a pixel corresponds to a specified combination of a value associated with each scatter pixel of the plurality of scatter pixels and a value associated with the transmission pixel corresponding to the scatter pixel. The final step of the method is that of displaying the combination image on a display device.
In accordance with other embodiments of the present invention, the beam of penetrating radiation may be a beam of x-rays, and the specified combination of value of each scatter pixel and value of the transmission pixel may be a linear combination of a function of the value associated with the scatter pixel and a function of the value associated with the transmission pixel. The specified combination may include a quadratic function of the value associated with the scatter pixel and a quadratic function of the value associated with the transmission pixel. Furthermore, the value associated with each scatter pixel may be a logarithmic function of an intensity of scattered radiation detected at the scatter pixel. Similarly, the value associated with each transmission pixel may be a logarithmic function of an intensity of transmitted radiation detected at the transmission pixel.